Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore's Law), which means that the number of devices on a chip doubles every two years. Today's semiconductor fabrication plants are routinely producing devices having 130 nm and even 90 nm feature sizes.
The desire for higher performance circuits has driven the development of high-speed sub-100 nanometer (nm) silicon-on-insulator (SOI) complementary metal-oxide-semiconductor (CMOS) technology. In SOI technology, metal-oxide semiconductor field-effect transistors (MOSFETs) are formed on a thin layer of silicon overlying a layer of insulating material such as silicon oxide. Devices formed on SOI offer many advantages over their bulk counterparts, including reduced junction capacitance, absence of reverse body effect, soft-error immunity, full dielectric isolation, and absence of latch-up. SOI technology therefore enables higher speed performance, higher packing density, and reduced power consumption.
There are two types of conventional SOI devices: partially-depleted SOI (PD-SOI) devices, and fully-depleted (FD-SOI) devices. Conventional PD-SOI MOSFET is one in which the body thickness is thicker than the maximum depletion layer width Wd,max, and a conventional FD-SOI MOSFET is one in which the body thickness is thinner than Wd,max. The conventional PD-SOI and FD-SOI devices are planar devices, i.e., they are formed in the plane of the wafer.
It is noticed that remarkable progress has recently been achieved in PD-SOI technology. Although PD-SOI devices have the merit of being highly manufacturable, significant design burdens are faced by its users because of floating body effects. In PD-SOI devices, charge carriers generated by impact ionization near one source/drain region accumulate near the other source/drain region of the transistor. When sufficient carriers accumulate in the floating body, which is formed right below the channel region, the body potential is effectively altered.
Floating body effects occur in PD-SOI devices because of charge build-up in the floating body region. This results in kinks in the device current-voltage (I-V) curves, thereby degrading the electrical performance of the circuit. In general, the body potential of a PD-SOI device may vary during static, dynamic, or transient device operation, and is a function of many factors like temperature, voltage, circuit topology, and switching history. Therefore, circuit design using PD-SOI devices is not straightforward, and there is a significant barrier for the adoption of PD-SOI technology or the migration from bulk-Si design to PD-SOI design.
One traditional way to suppress floating body effects in PD-SOI devices is to provide an extra electrical connection to the body by adding a contact to the body for collection of current due to impact ionization. Various methods of making a contact to the body of a SOI transistor are known, but various disadvantages are known to be associated with these methods. One method for the suppression of the SOI floating-body effects is to use a linked-body device structure. However, the method is limited by a high body contact resistance.
Blake et al., in U.S. Pat. No. 4,946,799, described a process for making a body node to source node connection, where a contact region of the same conductivity type as the body node is formed within the source region in a self-aligned fashion, thus eliminating the floating body effects. In U.S. Pat. No. 6,387,739 issued to G. E. Smith III et al., a method for forming a body contact structure for SOI transistor is described.
Another way of avoiding floating body effects in SOI devices is to adopt a fully-depleted SOI (FD-SOI) technology. FD-SOI devices do not suffer from floating-body effects due to the fact that the body is fully-depleted. FD-SOI technology is therefore design-friendly since floating-body effects need not be accounted for in circuit design.
In a FD-SOI technology, devices with a low body-doping and/or a thin body thickness are used. Additionally, for good control of short-channel effects in ultra-scaled devices, the device body thickness is usually reduced to less than one third of gate length. Such a thin body thickness would require raised source/drain technology for series resistance reduction. However, raised source/drain formation, currently performed by selective epitaxy, is immature, expensive, pattern-density dependent, and may result in reduced manufacturing yield. In addition, SOI substrates with uniform ultra-thin Si films, as required for the manufacture of FD-SOI devices with ultra-thin body, are currently unavailable. Non-uniformity of the Si film thickness will result in significant fluctuations in the device characteristics and negatively impact the ease of manufacture.
U.S. Pat. No. 6,222,234, issued to K. Imai, describes a method for the fabrication of FD-SOI and PD-SOI devices on the same substrate. U.S. Pat. No. 6,414,355 issued to An et al. described the structure of silicon-on-insulator chips with an active layer of non-uniform thickness. U.S. Pat. No. 6,448,114 issued to An et al. described several methods of forming silicon-on-insulator chips with an active layer of non-uniform thickness. In these three patents, a SOI substrate with two different silicon film thicknesses is provided, where the FD-SOI devices reside in a region with a thinner silicon film, and the PD-SOI devices reside in a region with a thicker silicon film.
FIG. 2 illustrates a prior art integration of FD-SOI and PD-SOI transistors in the same chip, where FD-SOI transistors 12 are formed in a thin silicon layer 14 and PD-SOI transistors 16 are formed in thick silicon layer 18. The silicon layers 16 and 18 are both formed directly on a buried oxide 20, which is directly on a silicon substrate 22. Active areas 24 within the silicon layers 16 and 18 are separated from one another by isolation regions 26.
Other techniques have also been used to enhance transistor performance. For example, strain may be introduced in the transistor channel for improving carrier mobilities. Therefore, strain-induced mobility enhancement is another way to improve transistor performance in addition to device scaling.
In one approach, strain in the channel is introduced after the transistor is formed. In this approach, a high stress film 32 is formed over a completed transistor structure 30, as shown in FIG. 1. The stressor 32, i.e., the high stress film, exerts significant influence on the channel 34, modifying the silicon lattice spacing in the channel region 34, and thus introducing strain in the channel region 34. As shown in FIG. 1, the stressor is placed above the complete planar transistor structure, which includes a source 34 and a drain 38 formed in silicon region 40. A gate 42 overlies channel region 34 and is separated therefrom by gate dielectric 44. Sidewall spacer 46 can be included adjacent gate 42. This scheme is described in detail in a paper by A. Shimizu et at, entitled “Local mechanical stress control (LMC): a new technique for CMOS performance enhancement,” published in pp. 433-436 of the Digest of Technical Papers of the 2001 International Electron Device Meeting. The strain contributed by the high stress film is believed to be uniaxial in nature with a direction parallel to the source-to-drain direction.